Fix duplicate instrument registration warning to only fire on real conflicts, and add smarter remediation

Spec requirement

Duplicate instrument registration (same name, at least one identifying field different) MUST still return a functional instrument and SHOULD emit a warning (sdk.md:976-1016). On a real conflict, producers SHOULD follow the specified remediation order: prefer the longer description, convert units/temporality where possible, otherwise warn and pass through (data-model.md:313-347, Producer recommendations, L326 specifically covers the "prefer the longer string" rule).

Current behavior

meter.rb:256-262 (permalink) (base create_instrument, inherited by the SDK Meter) returns a working new instrument and warns, but it warns on any re-use of an already-registered name, even when all identifying fields (kind/unit/description) are actually identical. This isn't a real conflict per the spec's own definition of distinct. It also doesn't attempt any of the spec's remediation strategies (longer description, unit/temporality conversion). It warns and lets both instruments export independently.

Suggested fix

Only warn when at least one identifying field actually differs between the new and existing registration. When it does differ, attempt the spec's remediation order (prefer the longer description; attempt unit/temporality conversion where feasible) before falling back to warn-and-pass-through.

Related rows in SPEC_COMPLIANCE_METRICS.md

SDK-36, DM-1

Start at metrics_api/lib/opentelemetry/metrics/meter.rb:256-262 and read the inherited create_instrument path in the SDK Meter alongside the linked SDK and data-model requirements. Done means identical registrations are warning-free, real conflicts still return a functional instrument, and the specified remediation or fallback warning behavior is covered by the relevant metrics tests.